The invention relates to a process and an installation for multiple coating, in which the material to be treated is passed through baths in which it is treated chemically, by electrodeposition and/or by electrophoresis.
Processes and devices of this type are used to provide objects which at least partially comprise a conductive material with a surface whose technical or optical properties are adapted to the specific intended use. This can be achieved by chemical or electrochemical treatment of the surface, for example chromating of the surface of a metallic workpiece, and by electrolytic or electrophoretic application of a topcoat.
The known wet processes involve a very high level of outlay, since the material to be treated has to be passed through a series of vessels which are arranged in succession and contain a very wide variety of treatment liquids, for example cleaning agents, electrolytes and electrodip coatings. This takes place in continuous throughput installations with conveyors which move the material being treated from dip tank to dip tank through a treatment line.
Since large pieces of material to be treated can be guided individually and small pieces, with a view to achieving a homogeneous surface, can only be guided with a limited bed height through the individual baths, conventional installations are extraordinarily complex and have a relatively low throughput.
A particular problem is that of achieving a surface treatment which is uniform in all parts, for example a full-area electrodeposition and/or coating without flaws. Flaws of this type result in particular at the contact points between the material being treated and the conveying means for transporting the material being treated through the individual baths.
The invention is based on the problem of improving the economics of the known processes and devices for multiple coating without impairing the surface quality of the material being treated.
The solution to this problem consists in a process in which trays holding the material being treated are guided, with the aid of a plurality of conveyors, along a circuit from treatment station to treatment station and through the individual bath vessels for coating by electrodeposition or electrophoresis as well as through any pretreatment and subsequent treatment vessels. This takes place in two separate circuits which the material being treated passes through in succession, while the trays in each case only pass through one circuit. This means that a tray change is required when the material being treated is transferred from one circuit into the other.
The material being treated is initially pretreated on an electrodeposition tray and is then provided with an undercoat, for example of a zinc-nickel alloy, by electrodeposition, followed by subsequent treatment, for example chromating, conversion coating or passivation. It then leaves the first circuit and is transferred to another tray or into the second circuit, in which a topcoat is applied by electrophoresis, and after the coating operation the material being treated is usually dried. After the drying, the coating trays can be unloaded, cleaned and then again supplied with the material being treated from the first circuit.
During the coating, the trays are preferably moved horizontally between the upper and lower anodes or cathodes; this may take place continuously. Preferably, however, the electrodeposition and coating trays are guided in steps at least through the individual bath vessels.
The process according to the invention can particularly advantageously be carried out in an installation which comprises a plurality of tray conveyors. These conveyors are arranged in two groups, in each case in such a way that they form a closed circuit for the trays. The two circuits are connected to one another via a tray changer in which the material being treated passes from the electrodeposition circuit into the coating circuit or is transferred from an electrodeposition tray to a coating tray which guides the material being treated through a bath vessel, a dryer, if appropriate with a cooling zone, and preferably through an unloading station and/or a cleaning station.
The basis of the tray conveyors in the bath vessels is preferably formed by a lifting and displacement frame which moves the trays in steps through the vessels. The drive required for this purpose may comprise a pushing cylinder and a lifting cylinder which, by means of a finger, engages in a slot in the lifting and displacement frame. In this way, it is possible for a plurality of trays which have been placed in each case on a support, for example bearing rails, to be picked up simultaneously and moved a distance forwards and then put back down again.
The trays preferably have lateral bearing arms, by means of which they are supported on bearing rails which simultaneously supply the current to the trays and therefore to the material being treated. Grippers which are connected to the lifting and displacement frame and with the aid of which the trays on the bearing rails are moved, for example pushed, from one position to the next position, are able to act on these bearing arms. To ensure a coating which is as uniform and continuous as possible, the trays preferably move between anodes or cathodes arranged above and below them during the coating by electrodeposition and/or electrophoresis.
To prevent the formation of arcs and resultant burning during contacting, the installation may be equipped with two groups of contacts which are each closed alternately and in this way ensure uninterrupted supply of current to the trays. To make this possible, the vessel may be equipped with two groups of contacts. One group is fixedly connected to the lifting and displacement frame and therefore moves with the frame from position to position, while the other group of movable contacts is arranged in a stationary position, for example fixed to the vessel.
A tray from a frame with cross-bars and an electrically conductive head wire with an exposed contact strip on which the material being treated rests is particularly suitable for the electrodeposition. The tray may consist entirely of an electrically conductive material; in this case, with the exception of the contact strip, it has an electrically insulating coating.
On the other hand, however, the trays may also have a sawtooth bearing edge for the material being treated. A tray of this type is described in German patent 41 42 997, the content of which is deemed to form part of the present description.
The pallet changer preferably has linear bearing means, which can be displaced along a bridge and are preferably arranged in a carriage, above a lifting bench or a tray conveyor.
The conveyor may also have a lifting and displacement frame, on which in each case one arm of a plurality of two-armed angled levers engages, the other arms of which levers are connected to one another via a linkage. The lifting and displacement frame as well as one of the angled levers may each be provided with a dedicated drive, for example may be connected to in each case one hydraulic cylinder. Gripper arms, which are connected to the lifting and displacement frame, for the trays and bearing rails, which are arranged in the vessel, for the trays may be provided with contact pins for receiving the trays, so that the trays are always energized either via the gripper arm pins or via the bearing rail pins.
In order for the trays to be introduced and removed, the bath vessels or their lifting and displacement frames are preferably equipped with a tray manipulator, which may be equipped with a displaceable tilting table and a holding-down device for the material to be treated.
Finally, the coating conveyor may also run through an unloading station with linear bearing means, which can be moved in between the bearing elements or cross-bars of the trays, and an unloading station rake which can be displaced along the linear bearing means, and/or a cleaning station with driven cleaning spatulas or displaceable cleaning heads.